Page 52 - Separation process principles 2

1.7 Component Recoveries and Product Purities 17

1.7 COMPONENT RECOVERIES AND are rank-listed by decreasing volatility, and hydrocarbons
PRODUCT PURITIES heavier (i.e., of greater molecular weight) than normal pen-
tane and in the hexane (C6)-to-undecane (Cll) range are
separation operations are subject to the conservation of lumped together in a so-called C6+ fraction. The three distil-
mass. Accordingly, if no chemical reactions occur and the lation columns of Figure 1.9 separate the deethanized feed
process operates in a continuous, steady-state fashion, then into four products: a Cs+-rich bottoms, a C3-rich distillate,
for each component, i, in a mixture of C components, the an iC4-rich distillate, and an nC4-rich bottoms. For each col-
molar (or mass) flow rate in the feed, njF), is equal to the umn, each component in the feed is partitioned between the
sum of the product molar (or mass) flow rates, nip), for that overhead and the bottoms, according to a unique split frac-
component in the N product phases, p. Thus, referring to tion or split ratio that depends on (1) the component thermo-
Figure 1.6, dynamic and transport properties in the vapor and liquid
phases, (2) the number of contacting stages, and (3) the rela-
tive vapor and liquid flows through the column. The split
fraction, SF, for component i in separator k is the fraction of
TO solve (1-1) for values of njP), from specified values of that component found in the first product:
njF), we need an additional N - 1 independent expressions
involving nip). This gives a total of NC equations in NC
unknowns. For example, if a feed mixture containing
C components is separated into N product phases, C(N - 1)
where n(') and n(F) refer to component molar flow rates in
additional expressions are needed. General forms of these
the first product and the feed, respectively. Alternatively, a
expressions, which deal with the extent of separation, are
split ratio, SR, between two products, may be defined as
considered in this and the next section. If more than one
stream is fed to the separation process, njF) is the summation
for all feeds.
Equipment for separating components of a mixture is de-
signed and operated to meet desired or required specifica- where n(2) refers to a component molar flow rate in the sec-
tions, which are typically given as component recoveries ond product. Alternatively, SF and SR can be defined in
andlor product purities. In Figure 1.9, the block-flow dia- terms of component mass flow rates.
gram for a hydrocarbon separation system, the feed is the If the process shown in Figure 1.9 is part of an operating
bottoms product from a reboiled absorber used to deethanize plant with the measured material balance of Table 1.5, the
(i.e., remove ethane and components of smaller molecular split fractions and split ratios in Table 1.6 are determined
weight) a mixture of refinery gases and liquids. The separa- from (1-2) and (1-3). In Table 1.5, it is seen that only two of
tion process of choice in this example is a sequence of three the four products are relatively pure: Cg overhead from the
multistage distillation columns. The composition of the feed second column and iC4 overhead from the third column. The
to the process is included in Figure 1.9, where components molar purity of C3 in the C3 overhead is (54.80/56.00) or

Column 1 Column 2 Column 3

Deethanized feed R
C,
Component Ibmollh
CzH6 0.60
C3b 57.00
iC4H8 171.80
nC4H,, 227.30
iC~H~z 40.00
~CSHIZ 33.60
c.5' 205.30 -
735.70
nCp -rich
Figure 1.9 Hydrocarbon recovery process.